This application is based upon Japanese Patent Application Nos. 2000-199443 filed on Jun. 30, 2000, and 2001-10682 filed on Jan. 18, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a sensor, and more particularly, to a structure of a pressure sensor assembly having a base with a lead and a sensor chip connected to the base through the lead so that the sensor chip is held by the base.
2. Description of the Related Art
A pressure sensor held by a base having external leads is disclosed in JP-A-7-209115, or in JP-A-7-243926, and is shown in FIG. 15 as a prior art.
The pressure sensor has a sensor chip 44 with one side on which a diaphragm 55 is formed as a sensing portion. The sensor chip 44 is fixed to a pedestal 66, and is mounted on a stem 111 as a base plate having leads 22. The sensor chip 44 is electrically connected to the leads 22 through wirings J1.
Recently, the pressure sensor is, however, required to be miniaturized. In case of the pressure sensor shown in FIG. 15, the stem 111 must have a predetermined area above which the wirings J1 are disposed as well as an area for mounting the sensor chip 44, so that a miniaturization of the sensor must depend on a miniaturization of the sensor chip 44.
This invention has been conceived in view of the background as described above and an object of the invention is to provide a pressure sensor held by a base with a lead, capable of miniaturizing a size thereof.
According to a first aspect of the present invention, a sensor chip is electrically fixed to leads through electrodes formed on a surface thereof so that a surface of the sensor chip faces a surface of a base directly. As a result, a predetermined area of a base for disposing a wiring thereon can be cut down.
Preferably, the sensor chip is fixed to the base by a face down bonding.
Preferably, the leads are disposed within an area corresponding to the sensor chip when the sensor chip is fixed to the leads.
Preferably, a difference in a liner expansion coefficient between the base and the sensor chip is 10 ppm/xc2x0 C. or less. More preferably, the difference is 6 ppm/xc2x0 C. or less. Furthermore preferably, the difference is 2 ppm/xc2x0 C. or less. As a result, influence to the sensor chip caused by the difference in the liner expansion coefficient between the base and the sensor chip is restrained. In other words, an output from the sensor chip is prevented from varying by the difference.
Preferably, the electrodes disposed on the sensor chip are ball-shaped bumps formed by a wire bonding method. The ball-shaped bumps can absorb variation in height of the leads by deforming themselves. Moreover, the ball-shaped bumps can absorb shock occurring in connecting the bumps to the leads.
Preferably, the electrodes are formed by a metal plating method. By this method, electrodes can be formed before the sensor chip is diced from a wafer, and can be changed in size easily.
Preferably, the ball-shaped bumps are formed on the metal electrodes.
Preferably, the electrodes are connected to the leads through an intermediate. The intermediate can absorb variation in height of the leads. The intermediate is, for example, conductive paste, conductive resin film or the like.
Preferably, a region of the sensor chip including portion where the electrodes are formed is covered with a protection member. In this case, Yong""s modulus is preferably 1 GPa or less to decrease stress applied to the sensor chip, which is caused by the protection member, so that the output of the sensor chip is not varied by the stress of the protection member so much. More preferably, the protection is gelatinized resin (gel) because it is soft.
Preferably, the protection member is a membrane, so that the output of the sensor chip is not varied by the membrane so much since it is thin.
Preferably, the protection member coats sides of the sensor chip, which are exposed by dicing the sensor chip from the wafer.
Preferably, the electrodes are arranged so that the electrodes do not surround a diaphragm formed in the sensor chip, so that stress caused by the base is prevented from transmitting to the diaphragm.
Preferably, a stress relaxation member is disposed between one of the electrodes and one of the leads.
For example, the stress relaxation member is a lead capable of elastic deformation. The lead has a weak portion, or an hourglass portion.
The sensor chip may be fixed to the leads through the electrodes formed on the surface thereof so that an extending direction of the leads is in parallel with the surface of the sensor chip.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings.